The measurable volume of a magnetic resonance tomography image is restricted in all three spatial directions because of physical and technical conditions, such as a restricted magnetic field homogeneity and a non-linearity of the gradient field for example. Thus an image volume, a so-called field-of-view (FoV), is restricted to a volume in which the above-mentioned physical features lie within a predetermined range of tolerance and thus a faithful image of the object under examination is possible with normal measurement sequences. This field-of-view is however significantly smaller, especially in the x and y direction, i.e. at right angles to a longitudinal axis of a tunnel of a magnetic resonance system than the volume restricted by the ring tunnel of the magnetic resonance system. With usual magnetic resonance systems a diameter of the ring tunnel typically amounts to 600 mm, whereas the diameter of the field-of-view normally used, in which the above physical features lie within the range of tolerance, amounts to approximately 500 mm.
The problem of not being able to produce a faithful image in the edge area of the tunnel of the magnetic resonance system is usually resolved for pure magnetic resonance images by the area of the object under examination not being arranged at the edge of the tunnel but where possible in the center of the tunnel, referred to as the isocenter of the magnetic resonance system. With hybrid systems, such as a hybrid system consisting of a magnetic resonance tomograph and a positron emission tomograph for example, known as an MR-PET hybrid system, it is however frequently of decisive importance also to determine structures in the edge area as precisely as possible. In an MR-PET hybrid system human attenuation correction is of decisive importance for example. Human attenuation correction determines the attenuation of the intensity of the photons emitted after interaction between positrons and electrons on their path through absorbent tissue to the detector and corrects the received signal by just this attenuation. For this purpose a magnetic resonance image is acquired which shows the complete anatomy of the object under examination in the direction of the high-energy photons emitted by the positron emission tomography. This means that the anatomy of the object under examination is also to be detected as accurately as possible in the edge area of the tunnel of the hybrid system. The structures located in this area are typically primarily the arms of a patient to be examined, which can be arranged in the edge area close to a tunnel inner wall of the hybrid system.
In the prior art, a method has thus been proposed by Delso et al, to compensate for the information missing as a result of the field-of-view limitation in the MR image, by segmenting the body contours using uncorrected PET data (G. Delso, et al, Impact of limited MR field-of-view in simultaneous PET/MR acquisition, Journal of Nuclear Medicine Meeting Abstracts, 2008; 49: 162P). Since the field-of-view of a magnetic resonance system is limited to a volume in which the magnetic field inhomogeneity and the non-linearity of the gradient field lies within specific ranges, various correction algorithms have been provided in the prior art in order to extend the field-of-view. Thus in Langlois S. et al, MRI Geometric Distortion: a simple approach to correcting the effects of non-linear gradient fields, Journal of Magnetic Resonance Imaging 1999, 9(6), 821-31 and in Doran et al, A complete distortion correction for MR images, I. Gradient warp correction, Phys Med Biol. 2005 April 7, 50(7), 1343-61 a gradient warp correction is proposed. Furthermore in Reinsberg et al, A complete distortion correction for MR images: II. Rectification of static-field inhomogeneities by similarity-based profile mapping, Phys Med. Biol. 2005 June 7; 50(11):2651-61 a corresponding B0 field correction is proposed.